The present invention relates to transporting and storing temperature sensitive cargo over long periods of time using a controlled climate in the space where the cargo is loaded. Climate control includes controlling the temperature of the cargo within a certain acceptable range. Controlling the temperature includes bringing the temperature of the cargo into the acceptable range (by refrigerating or heating) and maintaining the temperature within that range. Climate control may also include controlling other parameters such as humidity and composition of the atmosphere.
Refrigeration is the process of removing heat from an enclosed space, or from a substance, and moving it to a place where it is unobjectionable. The primary purpose of refrigeration is lowering the temperature of the enclosed space or substance and then maintaining that lower temperature.
One commonly used refrigeration technique is the vapor-compression cycle. The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems.
A refrigerated container or reefer is a shipping container used in intermodal freight transport, including rail, ship and truck, where the cargo is refrigerated (chilled or frozen) for the transportation of temperature sensitive cargo. A reefer will usually have an integral refrigeration unit.
The reliability of the refrigeration unit is of paramount importance. The temperature of temperature sensitive cargo should be kept within predefined limits. Some cargo must be maintained frozen, and the temperature of any part of the frozen cargo must be kept below a predefined freezing temperature which depends on the cargo, e.g. below −18 degrees C. or lower, while other cargo and commodities such as fresh fruit and vegetables should be kept chilled to stay fresh, but not frozen. For chilled fruit and vegetables there is a lowest acceptable temperature below which the commodity will begin degrading and loose its freshness. Such temperature is dependent upon the type of commodity.
Refrigerated containers have a refrigeration system and one or more evaporator fans for extracting return air from the container. The extracted return air is blown past the evaporator to refrigerate the air, and the refrigerated supply air is blown as into the container where it circulates and exchanges heat with the cargo and/or with the walls of the container before returning to the evaporator.
When a cargo has been loaded into a refrigerated container at ambient temperature, the cargo must be refrigerated to obtain a target temperature, also referred to as the set-point temperature. The process of bringing the temperature of the cargo down to the set-point temperature is usually referred to as pull-down.
Traditionally, in pull-down mode the compressor operates at or near its full capacity, and the evaporator fan is operated at high speed to circulate refrigerated supply air in the container, in order to achieve the maximum refrigeration effect and to have the cargo refrigerated in a short time.
The energy required for circulating the air in the container is ultimately dissipated as heat in the container due to friction. Depending on whether the evaporator fan is operated in low speed mode or in high speed mode it delivers from a few hundred watts or up to a few kilowatts (kW) which is dissipated as heat in the container. This energy adds to the energy that enters the container from the ambient and the heat that is generated by the cargo itself, all of which must be removed by the refrigeration system. Assuming idealized and optimistic efficiencies of 100% of both the evaporator fan and the compressor, for each kW consumed by the evaporator fan another kW will be consumed by the compressor to remove the friction energy from the container and its cargo.
It is an object of the invention to save energy during pull-down and to have a short pull-down period.